linux-kernel - Re: [PATCH RFC/RFT v2 4/4] riscv: Stop emitting preventive sfence.vma for new userspace mappings with Svvptc

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Date: Thu, 1 Feb 2024 16:03:48 +0100
From: Andrea Parri <parri.andrea@...il.com>
To: Alexandre Ghiti <alexghiti@...osinc.com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ved Shanbhogue <ved@...osinc.com>, Matt Evans <mev@...osinc.com>,
	Dylan Jhong <dylan@...estech.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	linux-riscv@...ts.infradead.org, linux-mm@...ck.org
Subject: Re: [PATCH RFC/RFT v2 4/4] riscv: Stop emitting preventive
 sfence.vma for new userspace mappings with Svvptc

On Wed, Jan 31, 2024 at 04:59:29PM +0100, Alexandre Ghiti wrote:
> The preventive sfence.vma were emitted because new mappings must be made
> visible to the page table walker but Svvptc guarantees that xRET act as
> a fence, so no need to sfence.vma for the uarchs that implement this
> extension.

AFAIU, your first submission shows that you don't need that xRET property.
Similarly for other archs.  What was rationale behind this Svvptc change?


> This allows to drastically reduce the number of sfence.vma emitted:
> 
> * Ubuntu boot to login:
> Before: ~630k sfence.vma
> After:  ~200k sfence.vma
> 
> * ltp - mmapstress01
> Before: ~45k
> After:  ~6.3k
> 
> * lmbench - lat_pagefault
> Before: ~665k
> After:   832 (!)
> 
> * lmbench - lat_mmap
> Before: ~546k
> After:   718 (!)

This Svvptc seems to move/add the "burden" of the synchronization to xRET:
Perhaps integrate the above counts w/ the perf gains in the cover letter?

  Andrea